The present invention relates to optical transmission channel with electrical connectors.
The term optical transmission channel is understood to mean the assembly formed by an electrooptical emitter connected by an optical cable to an optoelectronic receiver. The emitter performs the electrical/optical conversion of the signal corresponding to the information to be transmitted, the optical cable carries the light signal supplied by the emitter and the receiver performs the reverse optical/electrical transformation of the light signal transmitted by the cable to restore it to electrical form. The respective conversions are obtained by end or terminal diodes. At the output, the emitter uses a photoemitting diode of the light-emitting diode type, or of the laser diode type. The receiver uses at the input a photodetecting diode of the PIN type or the APD type (avalanche photodiode). Each of these diodes is accurately positioned relative to the end face of the cable associated therewithin in order to obtain a good optical coupling and good mechanical characteristics.
According to the most widely used constructions, the optical coupling at the end of the cable is carried out with the aid of appropriate optical connectors, so as to facilitate the connection - disconnection operations for the purpose of fitting, removing or changing the optical cable, or for disconnecting the latter from the emitter and/or the receiver.
According to other known constructions, the cable connection - disconnection level is moved upstream of the emitting diode and downstream of the receiving diode. The terminal optical connectors can thus be replaced by electrical connectors making it possible to connect the electrodes of the terminal diodes to the corresponding circuit. Thus, it is possible to benefit from the advantages of this type of connection, while avoiding the disadvantages of optical connectors. Thus, the repeated handling of optical connectors leads to mechanical wear, which rapidly brings about a loss of power and a modification to the pass band of the optical conductors or conductors forming the cable (one fiber or a bundle of fibers) of the emitter - receiver link in question. However, in the case of electrical connectors, the aging of the connector is very slow and only leads to a slight increase in the ohmic resistance. This increase is so low that it has virtually no effect on the transfer function of the link. As a function of these solutions, the ends of the cable are terminated by a module including the terminal diode optically coupled to the cable and connected to an electrical connector. The optical coupling is obtained by an arrangement ensuring the relative mechanical positioning of the elements without actually using optical connectors. Electrical connector is of the coaxial or multipin type.
These constructions with terminal electrical connectors for the cable operate satisfactorily when the frequency band used is relatively low, not exceeding a few MHz for the highest frequencies. However, beyond this level, these lengths of the connections connecting the terminal diode to the corresponding electronic circuit across the terminal electrical connector have an influence which increases with the frequency and which modifies the transfer function of the system. If it is wished to obtain higher performance systems, whose high frequencies can easily reach several hundred MHz or even exceed 1 GHz, it is found that each of these components, i.e. diode or fiber, has an increasing influence on the response of the link. For example, for an optical fiber of type 50/125 (core diameter/sheath diameter in microns) and on considering elements of fibres with the same length, the high frequency transmitted can easily vary in the range 200 MHz to 1.2 GHZ on changing the cable element, i.e. in a ratio ranging from 1 to 6.